Re: Solaris ZFS on Linux [Was: Re: the " 'official' point of view"expressed by kernelnewbies.org regarding reiser4 inclusion]

Avi Kivity
2006-08-01T07:25:00Z

Theodore Tso wrote:
>
> Ah, but as soon as the repacker thread runs continuously, then you
> lose all or most of the claimed advantage of "wandering logs".
> Specifically, the claim of the "wandering log" is that you don't have
> to write your data twice --- once to the log, and once to the final
> location on disk (whereas with ext3 you end up having to do double
> writes). But if the repacker is running continuously, you end up
> doing double writes anyway, as the repacker moves things from a
> location that is convenient for the log, to a location which is
> efficient for reading. Worse yet, if the repacker is moving disk
> blocks or objects which are no longer in cache, it may end up having
> to read objects in before writing them to a final location on disk.
> So instead of a write-write overhead, you end up with a
> write-read-write overhead.
>

There's no reason to repack *all* of the data. Many workloads write and
delete whole files, so file data should be contiguous. The repacker
would only need to move metadata and small files.

--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.